This invention relates to a tablet unit and, in particular, to a tablet unit using magnetic wires made of an amorphous material for detection of a coordinate.
A conventional tablet unit adopts a coordinate detection system generally classified into an electrostatic coupling system or a magnetic system. In place of these systems, a novel tablet unit recently proposed makes use of the Matteuci effect of an amorphous magnetic wire (hereinafter simply referred to as a magnetic wire). Such a novel tablet unit is proposed in an article entitled "New Tablet Utilizing Matteuci Effect of Amorphous Magnetic Wires" and contributed to Magnetics Research Conference of Institute of Electric and Electronic Engineering (MAG-91-158, Aug. 23, 1991).
As will later be described in detail, the tablet unit disclosed in the above-mentioned article makes use of the Matteuci effect that a magnetic wire generates a pulse voltage when a magnetic field is excited by an alternating current in a direction perpendicular to the magnetic wire. The tablet unit comprises, as a coordinate detector, a plurality of magnetic wires arranged on a tablet board along a X-axis direction and a Y-axis direction in a matrix fashion. In the following description, the magnetic wires parallel to an X axis are called X-axis magnetic wires while the magnetic wires parallel to a Y axis are called Y-axis magnetic wires. Each of the X-axis and the Y-axis magnetic wires has one end connected to a switching circuit successively supplied with selection signals from a selection circuit for selectively putting the X-axis and the Y-axis magnetic wires into an electroconductive state. The other end of each of the X-axis and the Y-axis magnetic wires is connected to a detection circuit. The detection circuit is for detecting a particular one of the X-axis magnetic wires and a particular one of the Y-axis magnetic wires when the particular ones are indicated as a coordinate by a coordinate indicator pen during the electroconductive state. On the other hand, the switching circuit supplies selection pulse signals successively to the X-axis and the Y-axis magnetic wires in a predetermined cycle to thereby carry out a scanning operation. The switching circuit includes a plurality of switching elements in one-to-one correspondence to the X-axis and the Y-axis magnetic wires.
Generally, the number of the magnetic wires are not less than fifty in each of the X-axis and the Y-axis directions. Accordingly, the total number of the switching elements exceeds one hundred for both the X-axis and the Y-axis directions. This results in a substantial increase of the cost. It is assumed here that the number of the magnetic wires in each of the X-axis and the Y-axis directions is equal to n while each selection pulse signal has a period T. In this event, a scanning period is equal to nT because all of the X-axis and the Y-axis magnetic wires are scanned one by one in the above-mentioned tablet unit. Accordingly, the scanning period inevitably becomes long with the increase of the number of the magnetic wires.